Prior to beginning a course of radiotherapy, volumetric images of the patient, and specifically the target region, need to be obtained so that a plan for the treatment can be constructed. The aim of the treatment plan is to establish how to apply the radiotherapy to the patient so that the target region receives the desired, lethal dose, whilst the surrounding healthy tissue receives as little dose as possible.
Radiotherapy is often delivered by a linear accelerator-based system, which produces a beam of high-energy x-rays and directs this toward a patient. The patient typically lies on a couch or patient support, and the beam is directed toward the patient from an offset location. During treatment, the beam source is rotated around the patient while keeping the beam directed toward the target point (the “isocentre”). The result is that the isocentre remains in the beam at all times, but areas immediately around the isocentre are only irradiated briefly by the beam during part of its rotation. By positioning (for example) a tumour at the isocentre, the dose to the tumour is maximised whilst the dose to surrounding healthy tissue is reduced.
In addition, the cross-section of the beam can be varied by way of a range of types of collimator, such as the so-called “multi-leaf collimator” (MLC) illustrated in EP 0,214,314. These can be adjusted during treatment so as to create a beam whose cross-section varies dynamically as it rotates around the patient.
Other aspects of the radiotherapy apparatus can also be varied during treatment, such as the speed of rotation of the source and the dose rate. Thus, there are a large number of variables offered by the apparatus in order to tailor the radiation dose that is delivered to the patient.
The volumetric images are therefore analysed to identify a target region into which a minimum dose is to be delivered, any sensitive regions such as functional organs for which a maximum dose must be observed, and other non-target regions into which the dose is to be generally minimised. This three-dimensional map must then be used to develop a treatment plan, i.e. a sequence of source movements, collimator movements, and dose rates which result in a three-dimensional dose distribution that (a) meets the requirements as to maximum and minimum doses (etc) and (b) is physically possible, e.g. does not require the source to rotate around the patient faster than it is physically capable.
This can be expressed as a mathematical problem in which the overall dose to healthy tissue must be minimised, subject to constraints as to the maximum dose to sensitive regions, the minimum dose to the target, and the various machine constraints such as the maximum rotation speeds, possible MLC shapes, etc. Although complex, the mathematical problem can be solved by one of a range of techniques (with varying efficiency) but this does require significant computing time.
In addition, courses of radiotherapy are usually fractionated. That is, they usually comprise several cycles of a short period of therapy (known as a fraction) followed by a recovery period. Unhealthy tissue (i.e. that which is the target of the therapy) takes longer than healthy tissue to recover from each dose of radiation. Therefore, by managing the therapeutic dose that is delivered in each fraction, as well as the length of the recovery period between each fraction, the unhealthy tissue can gradually be destroyed while the healthy tissue survives.
As a course of radiotherapy can last several weeks or longer, it is possible if not likely that the target region will move and/or change shape during the course of the treatment. This can mean that the original treatment plan becomes ineffective, as it was based on a different three-dimensional pattern of regions. The consequence of this is that the target tissue may receive a lower dose than intended and healthy tissue may receive a higher dose than desired. To counteract this, new images of the patient and target region can be taken before the start of each fraction, and the treatment plan re-calculated to compensate for any movement of the target region.
It is preferable for this inter-fraction imaging to take place with the patient in the same position as they will be in during treatment. To that end, the patient needs to stay in the same position during imaging, during the period while the treatment plan is being updated, and also during the course of the treatment. This can be some time, increasing the potential discomfort of the patient. A reduction in the time taken to complete this process would be beneficial both to the patient and to the facility operating the radiotherapy apparatus, who could then treat more patients than before in the same period of time.
U.S. Pat. No. 7,593,505 discloses a method in which a library of previously accepted treatment plans is used to speed up creation of a new treatment plan at the start of the planning process. European patent EP1238684 discloses a method in which the treatment plan is updated before each fraction by combining new image information with an existing approved plan for the same patient. However, both of these methods still take a relatively long time to compute.